Polyalkylene terephthalates are used for manufacturing fibers, films and molded articles. They have excellent physical properties such as high wear resistance, durability, and high dimensional accuracy due to their partly crystalline structure and they are, therefore, particularly suitable for the manufacture of heavy duty molded articles. The mechanical properties can be further improved by incorporating reinforcing materials such as glass fibers (British Pat. No. 1,111,012, U.S. Pat. No. 3,368,995 and German Auslegeschrift No. 2,042,447).
Polyethylene terephthalate (PET) is particularly suitable for the manufacture of fibers, filaments and sheets, but it is hardly suitable for injection molding because of high molding temperatures (approximately 140.degree. C.) and relatively long molding times required. These stringent processing conditions prevent the use of polyethylene terephthalate for injection molding in spite of its high rigidity and heat distortion temperature. In particular, the high mold temperature required is beyond the equipment capabilities of many mold shops which have only water heated molds. Such molds are normally capable of temperatures no higher than about 110.degree. C. Thus, wider use of this polymer is restricted not only by the economics of its long processing cycle but also by the inability of many potential end users to process the resin at all.
Although polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT) require shorter molding times and lower molding temperatures (approximately 80.degree. C.) owing to their higher rate of crystallization, they are inferior to polyethylene terephthalate in their physical properties, particularly in their heat distortion temperature.
High crystallinity ensures hardness, dimensional stability and resistance to distortion even at elevated temperatures. For optimum properties high crystallinity must be arrived at as rapidly as possible. Thus, in polyethylene terephthalate crystallization can be improved and accelerated by nucleation with finely divided inorganic solids (Netherlands Patent Application No. 65.11744). Generally the maximum degree of crystallinity obtainable for PET gives an energy of crystallization of about 14 cal/g and satisfactory mechanical properties are usually obtained when the crystallization energy is in excess of about 6 cal/g. The injection molding cycle time which governs the economics of the process depends on the time interval for which the cast must stay in the mold. To obtain a satisfactory degree of crystallization these cycles must be uneconomically long even at high molding temperatures and, therefore, prevent the wide use of polyethylene terephthalate in the manufacture of injection molded articles.
An object of the present invention is the provision of polyalkylene terephthalates compositions which have a higher crystallization velocity and higher crystallinity than those already known. A further object of the invention are polyalkylene terephthalates compositions which can be molded more rapidly and at lower temperatures than has been possible heretofor. An especial object is the provision of polyethylene terephthalates compositions (PET) which can be molded at temperatures below about 120.degree. C. and still retain a high degree of crystallinity.
In such polyester compositions the degree of crystallinity required for high dimensional stability is achieved more rapidly so that the injection molding cycles are substantially shortened. Also, the molding temperature can be lower without impairing the crystallization behavior. The injection molded casts, therefore, cool more rapidly and the residence time in the mold is, therefore, also reduced.